In operation of military vehicles it is often desired to reduce the airborne exterior noise signature in order to avoid possible detection of the vehicle by enemy forces. At high vehicle speeds the track and suspension noise is a predominant part of the total vehicle noise output. At lower vehicle speeds, or at standstill, the engine exhaust noise is usually the principle noise source. The present invention is directed to a noise reduction mechanism for the engine exhaust. The mechanism is designed to be especially effective at low engine speeds, which is from idle up to about 1000 r.p.m. or slightly beyond, for many vehicles.
In one embodiment of the invention the mechanism includes a flow-throttling element arranged in a vertical pipe that forms the exit opening from a conventional engine muffler. The flow-throttling element has a central passage therethrough that provides a continuous path for escape of combusted gases from the muffler at all engine speeds. At low engine speeds the weight of the flow-throttling element causes the element to close a main annular port or passage, thus confining the entire flow to the smaller central passage; which provides effective muffling and lessened total emitted noise. At high engine speeds the greater exhaust flow forces the flow-throttling element upwardly to a position wherein the main annular port is opened, thereby avoiding back pressure effects which could otherwise occur at high engine speeds. The noise reduction effect at high engine speeds is thereby lessened.
I am aware of U.S. Pat. No. 3,703,937 which discloses the use of a movable flow-throttling element in an engine exhaust system. However, in that patented arrangement the flow-throttling element is normally biased toward an open condition; at high engine speeds the patentee attempts to utilize aerodynamic forces in the flowing gas to lift the throttling element to a closed position wherein the element acts as a reflecting surface for an oncoming pressure wave. The patented system is said to be especially adapted to two cycle engines for more effectively preventing undesired escape of fresh charge gases with the exhaust gases during exhaust-charging transition period in the engine cycle. I believe my throttling system is conceptually different than that of the system shown in U.S. Pat. No. 3,703,937.
The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without payment to me of any royalty thereon.